This invention relates to a steam generator and a method of operating same in which heat is generated by the combustion of fuel in a plurality of fluidized beds.
Steam generating systems utilizing fluidized beds as the primary source of heat generation are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The heat produced by the fluidized bed is utilized to convert water to steam which results in an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is commonly referred to as a bubbling fluidized bed in which a bed of particulate materials is supported by an air distribution plate, to which combustion-supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. In a steam generator environment, the walls enclosing the bed are formed by a plurality of heat transfer tubes, and the heat produced by combustion within the fluidized bed is transferred to water circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, for separating water from the steam thus formed which is routed to a turbine or to another steam user.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turndown afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a circulating fluidized bed process. According to this process, fluidized bed densities between 5 and 20% volume of solids are attained which is well below the 30% volume of solids typical of the bubbling fluidized bed. The formation of the low density circulating fluidized bed is due to its small particle size and to a high solids throughput, which require high solids recycle. The velocity range of a circulating fluidized bed is between the solids terminal, or free fall, velocity and a velocity beyond which the bed would be converted into a pneumatic transport line.
The high solids circulation required by the circulating fluidized bed makes it insensitive to fuel heat release patterns, thus minimizing the variation of the temperature within the steam generator, and therefore decreasing the nitrogen oxides formation. Also, the high solids loading improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
However the circulating fluidized bed process is not without problems, especially when used in a steam generation environment. For example, it normally lacks a method of independently controlling the outlet temperature of the reheat as compared to the temperature of the main steam and/or superheat, especially when it is necessary to heat both of these fluids to temperatures of 950.degree. F. or higher and maintain these temperature levels over a wide control range without excessive attemperation.